Many electrically powered systems, such as adjustable speed drives (ASDs) for subsea applications in connection with, for example, oil and gas production, utilize semiconductor switching elements, such as insulated-gate bipolar transistors (IGBTs). For instance, IGBTs may be used in DC-AC converters. In subsea applications where IGBTs and other components are not accessible once a device has been arranged at the seabed, it is desirable to have access to information regarding changes in the state of the IGBTs, such as changes due to aging effects, so that an expected lifetime may be predicted and appropriate measures may be taken.
The on state voltage of the IGBT is the voltage (Vce) across the IGBT, from collector to emitter, when the IGBT is conducting current, and the IGBT is in so called saturation mode, meaning that the IGBT is operating as a switch. This is the normal operation for an IGBT. This voltage depends on several factors. The main factor that is dynamic (i.e., not decided by the physical construction of the device) is the current flowing through the IGBT. Increasing current means increasing voltage. Typical values for a saturated IGBT may be from 0.5V to 2.5V. Another factor that determinates the voltage is the aging of the IGBT. As the IGBT ages, the on state voltage Vce increases. The difference in the voltage due to aging is typically a few hundreds millivolts throughout the lifetime.
It is customary to provide some kind of voltage measurement/estimation circuit in order to be able to determine whether an IGBT is out of saturation or not. The Vce in the off state of the IGBT (i.e., the IGBT is not conducting any current) may be very high, and the measurement circuit needs to be protected from this voltage. This is often done by inserting a diode that protects the measurement circuit. Then, sending a current through the diode and the IGBT, the voltage between collector and emitter (Vce) in on state may be estimated by measuring the total voltage drop over the diode and Vce. The voltage of the diode may be estimated, and then withdrawn, and Vce remains.
A practical circuit 100 for this purpose is shown in FIG. 1. As shown, the current source is exchanged with a voltage source Vs and a series resistor R1. This has two reasons, the first being that when the IGBT is off, the current from a current source would have nowhere to go, except into the measurement circuit. The measurement circuit is by nature a high-impedance circuit, and therefore the current source will set up a high voltage, which may damage the measurement circuit. The other reason is that voltage sources are more available. When a voltage source Vs is used, the measurement of the voltage Vm will then be over the resistor R1 instead, thus the voltage Vce may be calculated by the formulaVs−Vm−V(D1)−Vce=0.
By using such a circuit 100, the voltage drop over the diode D1 is not measured, and is therefore uncertain. The voltage drop over the diode D1 is non-linear both with regard to the current flowing through the diode D1 and the ambient temperature. Therefore, a Vce measurement based on the circuit 100 may not provide sufficient precision for determining the aging of the IGBT.
There may thus be a need for an improved way of measuring a voltage drop across a semiconductor switching element that provides sufficient precision to determine aging of the semiconductor switching element.